Oil sand deposits such as those found in the Athabasca Region of Alberta, Canada, generally comprise water-wet sand grains held together by a matrix of viscous heavy oil or bitumen. Bitumen is a complex and viscous mixture of large or heavy hydrocarbon molecules which contain a significant amount of sulfur, nitrogen and oxygen. Oil sands processing involves extraction and froth treatment to produce diluted bitumen which is further processed to produce synthetic crude oil and other valuable commodities. Extraction is typically conducted by mixing the oil sand in hot water and aerating the resultant slurry to promote the attachment of bitumen to air bubbles, creating a lower-density bitumen froth which floats and can be recovered in a separator such as a gravity separator or cyclonic separator. Bitumen froth may contain about 60 wt % bitumen, about 30 wt % water and about 10 wt % solid mineral material, of which a large proportion is fine mineral material. The bitumen which is present in a bitumen froth comprises both non-asphaltenic material and asphaltenes.
Froth treatment is the process of eliminating the aqueous and solid contaminants from the bitumen froth to produce a clean bitumen product (i.e., “diluted bitumen”) for downstream upgrading processes. The bitumen froth is diluted with a hydrocarbon solvent to reduce the viscosity and density of the oil phase, thereby accelerating the settling of the dispersed phase impurities by gravity or centrifugation. Either a paraffinic or naphthenic type diluent may be used. Examples of paraffinic type diluents include C4 to C8 aliphatic compounds and natural gas condensate, which typically contains short-chained aliphatic compounds and may also contain small amounts of aromatic compounds. Examples of naphthenic type diluents include toluene (a light aromatic compound) and naphtha, which may be comprised of both aromatic and non-aromatic compounds. The difference in the bitumen produced by use of either a paraffinic or naphthenic type diluent can be attributed largely to the presence of aromatics. Aromatics have the ability to hold asphaltenes in solution, whereas paraffinic type diluents cause asphaltene precipitation.
Use of paraffinic type diluents results in a relatively low bitumen recovery (generally about 90%), but in a bitumen product which is dry, light, and has a relatively low water and solids concentration (less than about 0.5 wt %). However, paraffinic type diluents precipitate a major proportion of asphaltenes from the bitumen froth, resulting in not only the trapping of water and solids by the asphaltenes, but also high bitumen losses (about 8%) to froth treatment tailings. There are both environmental incentives and economic incentives for recovering all or a portion of this residual bitumen.
In comparison, the use of naphthenic type diluents results in a relatively high bitumen recovery (generally greater than about 98%), but in a bitumen product which has relatively high water (about 2 to 4 wt %) and solids (about 0.5 to 1.0 wt %) concentrations. The combined water and solids concentration typically is greater than about 2.5 wt %. Due to the level of contamination which pose fouling and corrosion problems, the diluted bitumen is not suitable for direct pipelining to conventional refineries, cannot be sold to the open market, and must be upgraded using processes such as a coker or hadrocracker. The upgraded products are then hydrotreated to produce synthetic crude oil. In order for the diluted bitumen to be marketable, it must meet the pipeline quality specifications, i.e. <0.5 vol % BS&W, density of 940 kg/m3 at 15° C. and viscosity of 350 cSt (mm2/s) at 6° C.
The inability to produce marketable diluted bitumen product from conventional naphtha-based processes is an impediment to the oil sands industry. The opening of future mines creates a potential scenario that the current bitumen processing capacity may be insufficient to handle the quantity of bitumen product. The ability to produce marketable fungible bitumen from conventional naphtha-based processes would greatly enhance the flexibility of production operations.